US2093780A - Modulation system - Google Patents

Description

Sept. 21, 1937. w. A. E. QUILTER MODULAT ION SYS TEM Filed Aug. 25, 1933 T I l I u I I I l l i l l I I r I l I INVENTOR MAL/AM A. 0 7474-19 ATT RNEY Patented Sept. 21, 1937 UNITED sr ATEN FEE MODULATION SYSTEM Application August 23, 1933, Serial No. 686,365 In Great Britain September 2, 1932 5 Claims.

This invention relates to modulated carrier wave transmitters and has for its object to provide an improved system capable of dealing with high degrees of modulation while maintaining a relatively high efficiency and substantial rectilinearity of response.

With modulated carrier wave transmitters of the so-called power amplifier type, i. e., of the type wherein the modulated carrier wave is amplified in a so-called power amplifier prior to radiation, it is very difficult to obtain more than a comparatively low power efiiciency in the power amplifier stages, the reasons for this being that the efficiency of conversion in those stages is a substantially rectilinear function of the grid excitation .and it is necessary that the grid excitation in the said stages be kept low in the carrier condition so as to allow for the increase in excitation which will occur due to modulation.

The present invention provides a system wherein these difficulties and disadvantages are avoided and wherein the grid excitation is maintained high under all conditions and consequently the overall power efficiency can be maintained high.

According to this invention a modulated carrier wave transmitter includes a power amplifier to whose input terminals the carrier wave only is applied and the output of this power amplifier is coupled to the load, e. g., to an aerial circuit, by means of one or more thermionic tubes whose internal resistance is varied in dependence upon the signals, 1. e., the modulating potentials.

The invention is illustrated in the accompanying drawing and shows schematically one arrangement in accordance therewith.

Referring to the drawing unmodulated radio frequency carrier voltages are applied in pushpull to the grids of two high frequency tubes V1, V2 vhaving a common split input circuit C. The grids of the valves V1, V2 are biased negatively from a source of potential D, the said grids being biased to the point of cut-off of anode current so that the amplifier constituted by the tubes V1, V2 operates as a so-called class B amplifier. With this arrangement and assuming an unmodulated carrier wave input at C, as required by this invention, the elficiency of conver sion from direct current power to high frequency power accomplished by the tubes V1, V2 may be as high as 70%.

The anodes of the tubes V1, V2 are coupled through coupling condensers B to a tuned tank circuit A consisting of an inductance shunted by a condenser, the center point of the inductance being earthed.

The load circuit is represented in the accompanying drawing at F as consisting of an inductance shunted by a condenser and a resistance in series. This load circuit may, of course, in

practice be any suitable load circuit, such, for

example, as an aerial.

Coupling between the load circuit F and the tank circuit A is accomplished by means of a coupling valve arrangement whose effective coupling co-eflicient is varied in accordance with modulated potentials applied at J. This coupling system consists of two thermionic tubes V3, V4 arranged back to back and providing a substantially purely resistive coupling which is effective both in the direction A to F and vice versa.

Low frequency modulated potentials, for example, voice frequency potentials, are applied at J to the primary of a transformer K having two secondaries. These secondaries apply modulating potentials as shown to the grids of the tubes V3 and V4 and, in order to exclude radio frequency potentials from the modulating transformer K, choke coils L are provided in series with the secondaries of the said transformer K. Of course, the source of modulated potentials connected at J may be and in practice generally will be constituted by the output of a chain of low frequency amplifiers, not shown.

The tubes V3 and V4 have their filaments heated by high frequency currents by means of coils G and G, which are coupled respectively to the inductance in the circuit A and the inductance in the circuit F. Alternatively the filaments may be directly connected across a few turns of the inductances in the circuit A and F respectively, or the filaments may be heated from any other suitable source of heating current provided that the said source is suitably insulated from ground or effectively insulated as regards radio frequency potentials by means of choke coils or the like.

When the modulating potentials from J on the grid of V3 swing the same in the positive direction V3 becomes more conductive and a path for radio frequency energy is provided. This path includes G, the anode of V3, the point on H ground and back to 1-1. When the modulating potentials on the grid of V4 cause V4 to be more conductive, a radio frequency path is also provided between A and F. This path is from'H to ground to H to G and the cathode of V4 to the anode of V4 and to the point on H.

In order that there may be no stray undesired reactive coupling between the circuits A and F the circuit A with its associated components G, V3 and the appropriate inductances L are enclosed in a screening box E and the circuit F with its associated components G, V4 and the remaining inductances L are similarly enclosed in a second screening box E.

Since there is no coupling between the circuits A and F other than via the substantially purely resistive coupling provided by the tubes V3 and V4, the amount of power transferred to F from A is dependent upon the efiective resistance of these coupling tubes. The effect of the application of signal potentials to the grids of the tubes V3 and V4 is, of course, to vary their effective resistances in accordance with signal potential so that modulated high frequency currents occur in the circuit F. Thus the high frequency unit comprising the tubes V1 and V2 works into a circuit, A, which has a resistance which is, in effect, varied by means of the variable load transferred through the signal varied coupling.

It will be seen that with an arrangement as above described the effective coupling to the aerial or load and consequently the effective load on the amplifier, will vary in accordance with the signal. Moreover if the point of maximum coupling as determined by the tubes V3 and V4, and consequently the point of peak modulation, is maintained always below a critical coupling point at which over-modulation may occur, the input to the aerial or load circuit, should vary in a linear manner in accordance with the signal input and thus provide good modulation.

Furthermore, the grid of the power amplifier tubes may be arranged to be excited up to the limits set in practice by considerations of harmonic distortion, i. e., the power amplifier may ordinarily be set for an efiiciency as high as about 70%.

It will be noted that the load on the rectifiers or similar devices supplying anode potential to the power amplifier tubes will at all times be proportional to the instantaneous value of the signal and this involves that the source of anode potential supply must be of good regulation both as regards audio frequency and direct current loads, if difficulties due to anode voltage variations are to be avoided.

These requirements can be met by employing as the source of anode potential apparatus having an inherently low impedance characteristic, e. g., the mercury vapor rectifiers now available having low voltage drop and good direct current regulation, the requirements as to audio frequency being met by providing the customary very large condenser ordinarily provided at the present time in power amplifiers. Alternatively any suitable source of anode potential may be utilized if there be associated therewith voltage stabilizing means for maintaining the applied anode voltage substantially constant.

A further; difiiculty which may arise is that, even at the point where the coupling tubes are operating on that part of the modulation cycle at which they are biased to cut-01f so that no current flows between the anode and cathode of said tubes, the inherent coupling between the anode and cathode of the tubes may be such as to leave a residual reactive coupling between the circuits A and F thus tending to prevent very deep or complete modulation. This difliculty can be avoided either by providing any known and convenient arrangements for neutralizing the self-capacities of the coupling tubes and/or the whole coupling circuit arrangement may be in eifect connected between points which are not much above earth potential as regards high frequency. In the specific arrangement above described it has been assumed that the coupling tubes in effect connect a point near the high potential end of the inductance in the output circuit A of the power amplifier with a point near the high potential end of the inductance in the aerial or load circuit F, but it is not necessary that the eifective connection be made between these high potential ends. The effect of making the eifective connection, as regards coupling, between points of lower potential is to lower the actual resistance required for coupling and to render the self-capacities of the coupling tubes less important relative to the useful and desired coupling impedances provided thereby. Thus, where it is required to proportion the eifective resistance of the coupling end to use tubes readily avail ble commercially as coupling tubes, or again where the coupling tubes have cathodes heated other than by radio frequency currents and it is accordingly desired to reduce the effective radio frequency capacity of the coupling tubes to ground, the coupling tubes may be connected to points such as the points H shown in the figure instead of as actually illustrated, these points, H, being so selected that the eifective resistances of the coupling tubes desired to be used is comparable or equal to the effective resistance of the coupling theoretically required.

Any alternative method of applying the modulating signal to the coupling tubes may be used, for instance, a resistance-capacity coupling circuit may be substituted for the low frequency transformers shown in the drawing.

Having thus described my invention and the operation thereof, what Iclaim is:

1. Means for applying linear modulation at signal frequency to a carrier wave comprising, a pair of thermionic tubes each having an anode, a cathode and a control grid, a tuned circuit connected between the control grids of said tubes, said circuit being tuned to the carrier wave and energized thereby, a source of potential connecting a point on said tuned circuit to the cathodes of said tubes, said source of potential being of such a value as to render said tubes conductive only for a portion of the carrier wave cycle, a circuit coupled between the anodes of said tube, said circuit being tuned to the frequency of the carrier wave, a load circuit, an additional pair of thermionic tubes, a connection between the cathode of one of said additional tubes and a point on said circuit connected between the anodes of said first named pair of tubes, a connection between the cathode of the other of said additional pair of tubes and said load circuit, a circuit connecting the anode of said last named tube of said additional pair of tubes to the cathode of said first named tube of said addi-,

tional pair of tubes, a circuit connecting the anode of said first named tube of said additional pair of tubes to the cathode of said last named tube of said additional pair of tubes, and a source of modulating potentials connected with the control grids of said additional pair of tubes.

2. A device as recited in claim 1 in which the cathodes of said additional pair of tubes are each connected with a heating circuit and in which one of said heating circuits is coupled to said circuit coupled between the anodes of said first named pair of tubes, while the other of said heating circuits is coupled to said load circuit.

3. A device as recited in claim 1 in which the circuit connected between the anodes of said first named pair of tubes and one of said additional tubes is enclosed in a conductive screen, while the other of said additional pair of tubes and said load circuit are enclosed in another conductive screen.

4. A device as recited in claim 1 in which radio frequency choking inductances are inserted in the connections between the source of modulating potentials and the control grids of the additional pair of tubes.

5. Means for applying linear modulation at signal frequency to carrier waves comprising, a pair of thermionic tubes each having an anode, a cathode and a control grid, a tuned circuit connected between the control grids of said tubes, said circuit being tuned to the frequency of the carrier wave to be modulated and energized thereby, a source of negative potential connected between the control grids and cathodes of said tubes, said source of negative potential being of such a value as to bias said tubes to be conductive for a half of the cycle of the carrier wave only, a tuned circuit connected between the anodes of said tubes, said tuned circuit including an inductance, a load circuit including an inductance, a pair of modulator tubes, a circuit connecting the cathode of one of said modulator tubes to the first inductance and the cathode of the other of said modulator tubes to the second inductance, circuits connecting the anode of said last named modulator to the cathode of said first named modulator tube and. the anode of said first named modulator tube to the cathode of said last named modulator tube, a source of modulating potential, circuits connecting said source of modulating potentials between the control grid and cathode of each of said modulator tubes, and radio frequency choking inductances in said last named circuits.

WILLIAM ALFRED EDWARD QUILTER.

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